Satellites in geostationary orbits (GSO's) have been widely preferred for several decades because of the economic advantages afforded by such orbits. In a geostationary orbit, a satellite traveling above the Earth's equator, in the same direction as that in which the Earth is rotating, and at the same angular velocity, appears stationary relative to a point on the Earth. These satellites are always "in view" at all locations within their service areas, so their utilization efficiency is effectively 100 percent. Antennas on Earth need be aimed at a CSO satellite only once; no tracking system is required.
Coordination between GSO's and with terrestrial services is facilitated by designated allocation of designated "slots" angularly separated according to service type.
Given the desirability of geostationary satellite orbits and the fact that there are only a finite number of available "slots" in the geostationary "belt," the latter has been essentially saturated with satellites operating in desirable frequency bands up through the Ku-band (up to 18 GHz). As a result, the government has been auctioning the increasingly scarce remaining slots. This has encouraged the development of complex and expensive new systems including those using low Earth orbits (LEO's), medium Earth orbits (MEO's), and higher frequencies, for example, the Ka-band (up to (approximately) 40 GHz).
Growth to higher frequencies is limited by difficult problems of technology and propagation, and expansion in satellite applications requires exploitation of the spatial dimension (i.e., above and below the GSO belt). A host of proposed LEO and MEO systems exemplify this direction.
The recently filed LEO and MEO system applications, however, introduce a significant problem. Frequency coordination and sharing are made difficult by the unstructured criss-crossings of the lines of sight of these systems. This has the potential of severely impeding effective spectrum use with nongeostationary orbits (NGSO) in general.
There has been no known prior effort to exploit coordinatable systems of inclined geosynchronous orbits (IGO's) in systematic manner, even though the unused domain of inclined geosynchronous orbits offers great potential for the coordinatable growth of satellite service.
The prior art that bears the closest resemblance to the disclosed invention is the original, 12-hour, elliptic orbit, Soviet communications satellites (Molniya 1A, April 1965) that shared a common ground track with three or four satellites per group. Over the years, a number of Cosmos satellites employed these highly elliptic Molniya orbits (44,000 kn apogee, 400 km perigee, inclined 63 degrees to minimize apsidal drift) for military purposes. Significant differences and limitations (compared with the present invention) of these 12-hour orbits include unavoidable intertrack crossings that prohibit coordinatable growth, low coverage efficiency for a given region, limited operating life due to the atmospheric drag attending a low-perigee orbit, and radiation belt tranversal. The 12-hour period also requires handoff at least four times daily. For these reasons, these orbits do not lend themselves to a large coordinated system with region focus.
Listings of current orbital parameters show many older satellites that have deteriorated into inclined geosynchronous orbits, but this is due simply to the exhaustion of station-keeping fuel.
While the various prior techniques function with a certain degree of competence, none discloses the advantages of the coordinatable system of geosynchronous satellite orbits of the present invention as is hereinafter more fully described.